We report on the frequency stabilisation of a Nd:YAG laser at 946 nm to the Hertz-level. The laser will be used for ultra-high resolution spectroscopy of the 5s(2) S-1(0) - 5s5p P-3(0) transition in In+ and will ultimately serve as a local oscillator of an optical frequency standard based on a single trapped indium ion. To resolve the extremely narrow S-1(0) - P-3(0) resonance (natural linewidth 0.82 Hz) at 237 nm, the frequency-quadrupled Nd:YAG laser radiation has to be frequency stable at the Hertz-scale for measurement times up to several tens of seconds. We obtain the frequency stability of the laser by locking it onto an external reference cavity of high finesse, placed on an active vibration isolation platform. (C) 2002 Elsevier Science B.V. All rights reserved.